“Sure,” Gustavo shrugged in typically low-key fashion as he finished his BBQ sandwich. Naturally, Otavio, sitting next to him, didn’t object. The brothers say they never disagree.

This scene unfolded on one of the key gathering spots of the Rose Fitzgerald Kennedy Greenway. That’s where Os Gemeos (“the twins”), famous in the street art world for creating towering cartoonish figures with bright colors and grimacing expressions, began work on their first Boston piece. Depending on weather, they’ll need a little over a week to craft the mural on a wall of a Big Dig ventilation building and a second, smaller piece on the Revere Hotel near Boston Common.

The pieces are part of the first solo museum exhibition in the United States for Os Gemeos. The Institute of Contemporary Art show, featuring paintings, mixed media works, and installations, opens Aug. 1.

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“For some people, this is kind of like the Rolling Stones coming and giving a free concert on the Greenway,” said Geoff Hargadon, the ICA supporter and street art enthusiast who had encouraged the city to find open space for the twins.

In that spirit, a steady stream of observers — admirers in the know, high school kids, workers breaking for lunch in Dewey Square — discovered the twins working on lifts. Their arrival Wednesday, in true street art fashion, was not telegraphed, though it couldn’t be kept completely secret because of a handful of state and local agencies that cooperated to allow the use of the ventilation building wall.

“It was really just a rumor,” said Sheryl Pace, 33, an artist who teaches at the Massachusetts College of Art and Design. “You can google this and you’re not going to find anything about this.”

A few days ago, she talked to local street artist and historian Caleb Neelon, who has known the Pandolfos for 15 years. Were they coming? He told her “absolutely not,” Pace said. Still, she had her suspicions.

Neelon, who showed up yesterday to hang out with the twins, shrugged when asked why he didn’t come clean.

“So you don’t have 50 people taking the bus from New York with little canvases to try to get these guys to paint,” said Neelon, coauthor of last year’s “The History of American Graffiti.”

Boston has never been viewed as a particular haven for artists like the twins, a perception validated for some in 2009 when Boston Police arrested artist Shepard Fairey outside the ICA as he attempted to attend the kickoff party for his first solo show. In the end, the majority of vandalism charges against Fairey were dismissed.

Nancy Brennan, executive director of the Greenway Conservancy, said she hopes the Os Gemeos project signals a wave of public art projects. She said a Conservancy working group plans to release a report this fall detailing proposals for putting up public art over the next five years.

“This is an untapped opportunity for artists in Boston,” said Brennan. “It’s a very fresh and contemporary place and I think we can have fresh opportunities because of that.”

Pace, who watched as work began on the mural, said she was impressed.

“You don’t have to pay money to go into a museum or feel intimidated to go into an art gallery,” said Pace, 33. “It’s for the commuters, the homeless, the youth working in the garden. It’s for everyone.”

That’s a philosophy shared by the brothers, who are 38. They have few requirements when they work other than that the Montana paint they use be from Spain, not Germany.

Around 10:15 a.m. on Wednesday, Gustavo, the bearded twin, sat in the shade against the wall twirling a paint brush like a drum stick as he waited for his brother, who was texting on his cellphone from across the park.

When asked whether it’s different to do work in public without permission, he shook his head.

“If you want to paint something, you do what you want to do,” he said. “We don’t see illegal or legal. The city — it’s just a big canvas.”

While artists who become successful often hire crew members to create and install their works — think Dale Chihuly or Tara Donovan — Os Gemeos wouldn’t dream of such a thing. That’s because the twins say they don’t know exactly what a piece will look like until they’re painting. This style of improvisation requires they be on the scene.

“We need to do it,” said Gustavo. “It’s very important, when you’re an artist, to touch and feel and be involved. We have people to help but we really need to be part of it.”

To that end, they got help with the Greenway mural only from a worker from their Brazilian studio and a preparator from the ICA.

A little before 11, the brothers began work. Gustavo wore white Nikes and a paint-splattered pair of brown corduroys, Otavio black sneakers, shorts, and a sweatshirt. They were on separate lifts and without any sort of walkie-talkies, typical of the twins, who say they’re able to communicate without speaking.

Within an hour, Otavio, with a paint brush on a long stick, had traced eyes, a mouth, and eyebrows high on the wall. Gustavo had sprayed part of the figure’s head with white paint. Pedro Alonzo, the curator who organized the Fairey show and the ICA art wall by Swoon last year, and has worked with the brothers in the past, rode up on his bicycle.

It wasn’t until that point that he knew the twins were creating a figure.

“They’re never going to tell you,” Alonzo said. “They just won’t. But I wasn’t worried. They’re extremely professional and totally dedicated to their work and in their public art, there’s never anything obscene or overly political.”

ICA Director Jill Medvedow said she also wasn’t concerned.

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The Institute of Contemporary Art opens a show on Aug. 1 featuring works of Os Gemeos, including “Back in the Days” (above).

“Commissions of new work do come with some uncertainty but it’s not uncertainty in a vacuum,” she said.

Os Gemeos certainly have a track record. They have been painting since their teens in the late 1980s. In the last decade, though, they’ve become more famous with their massive murals popping up around the world, whether on a school building in New York’s Chelsea neighborhood or on a castle in Scotland. They’ve also gone legit. In 2008, the twins sat down with public officials in their native Sao Paolo, leading to the creation of authorized works around the city.

The brothers also create works for inside, from ink drawings to eye-popping installations made up of glittery, surrealist sculptures of faces, cars, and boats.

The one constant is that they never work alone.

Why?

“Because we are just one artist,” said Otavio.

“It’s the same way we would play as children, but now on a big scale,” he continued, explaining during a lunch break Wednesday. “I know what he wants to do and he knows what I want to do. We talk a lot, only with telepathy.”

They never disagree on where a work is headed, which is notable since the brothers say pieces change in process. The figure that began to emerge Wednesday in Dewey Square, for example, is still a work in progress. The twins, known for the stunning details that adorn their figures, say they aren’t sure what patterns will emerge as they go. Come back in a few days, said Gustavo.

“It’s too early. Before we start on the pattern, we get inspired by everything we see in the city,” he said before pointing to a window in a building bordering the square and then an old roof in the distance.

“Surprise is the best thing in life,” Otavio added. “It makes life exciting.”

It was Alonzo who scouted out Boston for a proper public canvas. A handful of buildings were considered, with the brothers’ first choice being the brick, Dainty Dot building on the edge of Chinatown. But the former factory building is being demolished, so they settled on the Dewey Square location. Alonzo also hooked the twins up with the Revere Hotel, feeling it would be perfect to feature Os Gemeos in a spot in the center of Boston’s night life.

The Dewey Square creation — which is being paid for by donations to the Greenway and also the ICA’s exhibition budget — will be up for about 18 months, which fits into Medvedow’s approach to public art.

“Because it’s temporary,” she said, “both the artist and the ICA can take a risk.”

From the sounds of the observers Wednesday, it’s hard to imagine anyone viewing the project as a risk.

Brandon Aguiar, an artist, arrived on Wednesday shortly after the twins began work. He was still there four hours later, as the figure began to take some shape with an arm visible at the bottom of the building face.

“I’m a big fan of theirs and I’ve never had the opportunity to see them work,” Aguiar, 22, said. “It’s so awesome that it’s happening right here.”